1. Field of the Invention
This invention relates to a process for sequestering carbon dioxide in concrete through the process of redirecting the exhaust from a concrete mixing truck combusting carbonaceous fuel into the mechanical mixing chamber of the truck and physically mixing with the fresh concrete.
2. Description of the Related Art
Anthropogenic carbon dioxide is being viewed as a pollutant with respect to its contribution to the levels of greenhouse gases in the atmosphere. There are many anthropogenic sources of carbon dioxide including the burning of carbonaceous fuels and the manufacture of cementitious materials containing calcium oxide using materials containing calcium carbonate as the raw material.
Sequestration of carbon dioxide from the atmosphere can occur by many biological, chemical and physical processes. One natural process of sequestration is the process called carbonation where carbon dioxide from the atmosphere chemically reacts with the calcium oxide or calcium hydroxide phases in concrete and forms calcium carbonate. This process is usually very slow and is dependent on many variables such as the concentration of carbon dioxide in the air, the age of the concrete, the availability of water, the surface area of the concrete exposed to the atmosphere, and the porosity of the concrete.
In the manufacture of concrete, the ingredients are mechanically mixed in a concrete mixing truck and exposed to the atmosphere in the process, which usually lasts for time periods measured in hours. However, the concentration of carbon dioxide is low, measured in the units of a few hundred parts per million. Therefore, there is little opportunity for substantial amounts of carbon dioxide to be reabsorbed and sequestered into the concrete mix during the short mixing period.
It has been reported that in 2000, carbon dioxide emissions reached 5.8 billion metric tons and that approximately 80% of annual emissions come from the burning of fossil fuels. Therefore, there have been efforts to limit man made carbon dioxide emissions from fossil fuel burning plants. U.S. Pat. No. 6,235,092 provides a discussion of a variety of processes that have been developed for removing a gaseous component (such as carbon dioxide) from a multicomponent gaseous stream (such as the exhaust gas stream of a coal burning electrical power generation plant). Selective adsorption by solid adsorbents and gas absorption are named as two example processes. This patent further mentions that gas absorption finds use in the separation of carbon dioxide from multicomponent gaseous streams. Other patents describing carbon dioxide sequestration methods include U.S. Pat. Nos. 6,648,949, 6,372,023 and 5,397,553.
Although these processes may be successful in-sequestering carbon dioxide, they can be energy intensive. Thus, there is continued interest in the development of less energy intensive processes for sequestering carbon dioxide from the exhaust gas streams from the combustion of carbonaceous materials. Patent Ser. No. 12/571,398 uses the carbon dioxide from an exhaust stream but for preformed concrete applications, not ready-mixed.
It is also known that the hardening of cement-containing materials can be accelerated by carbonation in which calcium hydroxide in the cement is transformed into calcium carbonate by absorbing carbon dioxide. Related processes are described in U.S. Pat. Nos. 6,387,174, 6,264,736, 5,965,201, 5,897,704, 5,690,729, 5,650,562, 5,518,540, 5,307,876, 5,051,217, 4,427,610, 4,362,679, 4,350,567, 4,117,060, 4,093,690 and 4,069,063, German patent application DE 4207235, Swiss patent application CH 644828, and Japanese patent applications JP 6263562 and JP 2018368.